Hematopoiesis, the generation of blood cells, is a vital and essential developmental phenomenon providing oxygen supply, immune surveillance and hormone circulation to many organisms. Consequently, the normal development and functioning of this system is critical for the viability and well being of many animal species, including humans; even minor disruptions of this process can result in debilitating and life threatening conditions such as anemias, cytopenias, hemophilias, leukemias and lymphomas. Yet, despite major advances over the years in deciphering the workings of this system in a wide range of organisms, many aspects of it still remain a mystery. The overall goal of this proposal is to establish the fundamental principles and events of hematopoiesis from the embryo to the adult at the molecular, cellular and whole organism level in mammals. Gfi-1 (growth factor independence-1) and Gfi-1b are transcription factors that play essential and decisive roles in mammalian hematopoiesis. Gfi-1 is required for maintaining functional integrity of adult stem cells in the bone marrow, the generation of neutrophils and appropriate lymphoid development. Gfi-1b is essential for the generation of the erythroid and megakaryocytic lineages and for proper differentiation of primitive (yolk sac derived) erythroid cells. Consistent with the decisive roles of these genes in normal hematopoiesis, they exhibit malignant properties when ectopically expressed in leukemias and lymphomas. In light of our recent findings linking Gfi-1 and Gfi-1b to chromatin modifiers LSD1 (lysine specific demethylase1), CoREST/Rcor1 (REST co-repressor), other Rcor homologs and the histone deacetylases HDAC1-2, we propose to explore the role of these co-factors in establishing transcriptional and epigenetic programs in conjunction with Gfi-1/1b. We will further determine how the individual and combinatorial actions of these proteins impact the formation, fate and function of blood cells from early ontogeny to the adult stage. Additionally, we will investigate the mechanism of selection and regulation of Gfi-1/1b gene targets by these proteins and their cofactors in multiple cellular contexts and determine the biological consequences of these processes. These experiments will produce valuable insights and information into the fundamental genetic and epigenetic programming of hematopoietic development in mammals. These insights will further serve as paradigms for understanding other developmental systems where LSD1 and Rcor proteins produce similar effects. Moreover, insights gained by studies into the normal function of these factors will illuminate our overall perspective of stem and progenitor cell biology, both under normal conditions and in hematological diseases.